First, you have to assume the level of efficiency. The more efficient the rider, the lower the oxygen consumption at any power output. So, Vayer may have assumed efficiency to be lower than the reality for Contador - short of measuring it, you'll never know.

Contador is likely to be very efficient. Not only has he been cycling at such a high level for a long time, but he has fantastic form on the bike. So not only is his "internal" efficiency amazing, but he also is likely to have less wasted effort than the average trained cyclist.